Microwave bandpass filters are two port devices which allow the reception of a predetermined band of frequencies and suppress all other frequencies. An ideal filter will accept frequencies within a frequency range which precisely matches the band width and center frequency of the communication spectrum desired. With that range, it will pass signals with almost no dissipation or distortion.
Current parallel coupled microstrip bandpass filters have a fixed center frequency. They employ spaced parallel coupled microstrip lines printed on a planar dielectric. Thus their structure is simple and inexpensive. However, these and other bandpass filters operating above 1GHz have only a fixed center frequency and bandwidth.
A tunable filter has an electronically adjustable center frequency which, when used correctly, will operate within a linear frequency range. Tunability allows the filter to be adjusted to the center frequency of the signal to be received and also allows reception of multiple signals in a multiplexing scheme and is usable in a number of microwave applications.